Cardiac Physiology

• What is a heartbeat?

  A single cardiac contraction
• How do the heart chambers contract?

  First the atria, then the ventricles
• What are the two types of cardiac muscle cells?

  Autorhythmic cells and contractile cells
• What is the role of autorhythmic cells?

  Control and coordinate the heartbeat
• What do contractile cells do?

  Produce contractions that propel blood
• What does the conducting system consist of?

  Pacemaker and conducting cells
• What is autorhythmicity?

  Ability to contract without external stimulation
• Where are pacemaker cells found?

  In the sinoatrial (S A) node
• What is the primary pacemaker of the heart?

  The sinoatrial (S A) node
• Where is the atrioventricular (A V) node located?

  At the junction between atria and ventricles
• What are the components of the conducting system?

  Internodal pathways, A V bundle, bundle branches, Purkinje fibers
• What is the pacemaker potential?

  Gradual depolarization of pacemaker cells
• What is the rate of action potentials for the S A node?

  60–100 per minute
• What is the rate of action potentials for the A V node?

  40–60 per minute
• What establishes the sinus rhythm?

  The S A node
• Why is the normal resting heart rate slower than the S A node rate?

  Parasympathetic stimulation slows the heart rate
• What happens after the S A node generates an action potential?

  The stimulus spreads to the A V node
• Why is the impulse delayed at the A V node?

  To allow the atria to contract first
• What are the steps of impulse conduction through the heart?

  1. S A node generates an action potential
  2. Stimulus spreads to A V node and atrial contractile cells
  3. Impulse is delayed at A V node
  4. Impulse travels along A V bundle to bundle branches
  5. Impulse spreads through Purkinje fibers to ventricles
• How does the impulse travel after the A V node?

  Along the A V bundle to bundle branches
• What stimulates the papillary muscles in the ventricles?

  The impulse from the AV bundle
• How do the ventricles contract?

  In a wave from apex to base
• What are the features of an electrocardiogram (E C G)?

  • P wave: depolarization of atria
  • Q R S complex: depolarization of ventricles
  • T wave: repolarization of ventricles
• What is bradycardia?

  Abnormally slow heart rate
• What is tachycardia?

  Abnormally fast resting heart rate
• What is an ectopic pacemaker?

  Abnormal cells generating higher action potentials
• What does an electrocardiogram (E C G) record?

  The electrical events in the heart
• How is an E C G obtained?

  By placing electrodes on the body surface
• What does the P–R interval indicate?

  Time from atrial to ventricular depolarization
• What does a long P–R interval indicate?

  Damage to AV node or pathways
• What does the Q–T interval measure?

  Time for ventricles to depolarize and repolarize
• What does a long Q–T interval indicate?

  Electrolyte disturbances or myocardial damage
• What happens during rapid depolarization in cardiac contractile cells?

  Influx of sodium ions through fast channels
• What occurs during the plateau phase of action potential?

  Calcium ions enter and keep the cell depolarized
• What is the refractory period?

  Time when the cell won't respond to stimuli
• What is the absolute refractory period?

  Cardiac cells cannot generate another action potential
• What is the relative refractory period?

  Cells respond only to strong stimuli
• How does the action potential duration in cardiac cells compare to skeletal muscle fibers?

  30 times longer in cardiac cells
• What role do calcium ions play in cardiac muscle contractions?

  Trigger release of calcium from the sarcoplasmic reticulum
• What is the energy source for cardiac contractions?

  Aerobic reactions breaking down fatty acids and glucose